The present invention relates to an electrophotographic printing apparatus, more particularly an improvement in the charging unit of an electrophotographic printing apparatus.
Electrophotographic printing apparatus can be found in facsimile machines, copiers, and, of course, printers. The charging unit charges a photosensitive drum, which is then exposed to light to form a latent electrostatic image. The image is developed by application of charged toner particles. The resulting toner image is transferred to paper or other printing media by an oppositely charged transfer unit. Toner that fails to be transferred is removed from the surface of the drum by a cleaning unit.
The charging unit of a conventional electrophotographic printing apparatus comprises a charging body such as a roller that turns in contact with the photosensitive drum, and a constant-voltage source that holds the charging body at a fixed potential. The potential difference between the charging body and the photosensitive drum, which is grounded, causes charge to be deposited from the charging body onto the surface of the photosensitive drum.
A problem in the conventional electrophotographic printing apparatus is that during the developing and transfer processes, a small amount of toner acquires a charge opposite to the normal toner charge. This oppositely charged toner is neither transferred to the printing media nor removed by the cleaning unit, but is attracted to the charging body by the potential difference between the charging body and photosensitive drum. Accumulation of such toner on the charging body increases the electrical resistance between the charging body and the surface of the photosensitive drum, thus reducing the amount of charge deposited on the drum surface during the charging process. The result of this inadequate charging is that toner adheres to unwanted portions of the photosensitive drum, causing printing defects such as fogging.